OBJECTIVE To assess the diurnal distribution of ventricular tachycardia (VT) and atrial fibrillation (AF) in critically ill patients. METHODS Prospective observational study (episode-based design) in an eight-bed medical/cardiological ICU at a university hospital that also admits postoperative patients. METHODS 98 consecutive patients with AF ( n=55) or ventricular tachycardia ( n=43). RESULTS There were a total of 218 arrhythmia episodes (83 AF, 136 VT). The time of arrhythmia onset was not evenly distributed. Circadian variation in VT but not AF onset was well represented by a sine wave function. Both VT and AF fibrillation showed a trough during the night. The distribution of VT and AF episodes did not differ significantly with or without analgosedation. CONCLUSIONS In critically ill patients the onset of VT and AF over 24-h is nonuniformly distributed. In VT this circadian pattern of occurrence is well modeled by a sine wave function peaking between noon and 2 p.m. The circadian pattern is less clear for AF. The circadian variation is seen irrespective of the presence of absence of analgosedation for both arrhythmias.